Liquid developer for electrophotography

ABSTRACT

A liquid developer for electrophotography comprising particles of an image forming material selected from dyestuffs, pigments and powdery metals dispersed in a composite dispersion medium comprising a liquid carrier of high electric resistance and an asphalt extract which is at least substantially completely dissolvable in the liquid carrier and which is obtained by extracting an asphalt with a saturated hydrocarbon liquid, in which the image forming material particles are charged negatively by the dispersion medium, thereby giving a reverse visible image in the case of a negative electrostatic latent image or a nonreverse visible image in the case of a positive electrostatic latent image.

United States Patent 1191 Morikawa et al.

[ LIQUID DEVELOPER FOR ELECTROPHOTOGRAPHY [75] Inventors: Hiroichi Morikawa; Keiichi Nagahashi; Tatsuo Aizawa, all of Osaka, Japan [73] Assignee: Mita Industrial Company, Ltd.,

Higashi-ku, Osaka, Japan [22 Filed: Oct. 16, 1972 [211 App]. No.: 297,712

Related US. Application Data [63] Continuation-impart of Ser. No. ll5,886, Feb. l6,

197 l abandoned.

[52] US. Cl. 252/62.1 [51] Int. Cl G03g 9/04 [58] Field of Search 252/621 [56] References Cited UNITED STATES PATENTS 3,438,904 4/1969 Wagner 252/62.l 3,554,946 l/l97l Okuno ct al. 252/62] June 28, 1974 2/l972 Okuno et al. 252/62,] 4/l972 Machida et al. 252/621 [5 7] ABSTRACT A liquid developer for electrophotography comprising particles of an image forming material selected from dyestuffs, pigments and powdery metals dispersed in a composite dispersion medium comprising a liquid carrier of high electric resistance and an asphalt extract which is at least substantially completely dissolvable in the liquid carrier and which is obtained by extracting an asphalt with a saturated hydrocarbon liquid, in which the image forming material particles are charged negatively by the dispersion medium, thereby giving a reverse visible image in the case of a negative electrostatic latent image or a non-reverse visible image in the case of a positive electrostatic latent image.

13 Claims, N0 Drawings LIQUID DEVELOPER FOR ELECTROPHOTOGRAPHY This application is a Continuation-ln-Part application of co-pending application Ser. No. 115,886 filed Feb. 16, 1971, now abandoned.

This invention relates to a liquid developer for electrophotography. More specifically, it relates to a liquid developer for electrophotography comprising particles of an image forming material such as dyestuffs, pigments and powdery metals dispersed in a composite dispersion medium in which the image forming material particles are charged negatively by the dispersion medium, thereby giving a reverse visible image in the case of a negative electrostatic latent image or a nonreverse visible image in the case of a positive electrostatic latent image.

It has been known that when particles of coloring materials such as dyestuffs and pigments are dispersed in a liquid medium of a high electric resistance, the coloring material particles are charged positively or negatively depending on the correlation between the kind of liquid medium and the kind of color material, and that an electrostatic latent image can be converted to a visible image by an action of the resulting suspension. It has also been known in the art that for the purpose of maintaining the charge of the dispersed pigment particles at a certain level, the pigment particles are covered with an alkyd resin, linseed oil or the like and the resulting coated particles are dispersed in a highly electrically insulating liquid. However, in a liquid developer of this type, the resin coating of the particles is gradually eluted out into the liquid during long-time usage or storage, with the result that a change in the polarity in the pigment particles occurs, forming precipitates of agglomerated particles of the coloring material or that degradation of electric properties of the liquid of a high electric resistance is' brought about. These defects are inevitable in conventional liquid developers of the above type.

In conventional known liquid developers for electrophotography it is generally difficult to uniformly negatively charge pigment particles. Some combinations of coloring materials with liquid media or of coloring material particles with liquid media and controlling agents have been proposed to produce liquid developers in which the coloring material particles are negatively charged. However, these liquid developers are poor in their durability effect and fail to give a distinct image when used for a long time.

A typical liquid developer of the foregoing type is illustrated in US. Pat. 3,554,946 to Okuno et al., the liquid developer being one which contains a pigment coated with a copolymer. The liquid developer is one which is prepared by adding to a pigment an electric charge polarity controlling agent which consists of a synthetic resin having a specifically defined chemical structure, the mixture being kneaded and dispersed either independently or together with a surface-active agent in a carrier liquid which consists of a hydrocarbon having a high electric resistance. The Okuno pa tent indicates that when a naturally-occurring rosin or asphalt is used in'combination with pigment particles such as carbon black which, by its nature, never exhibits a distinct electric charge polarity, it is difficult to control the electric charge of the particles requiring the additional use of an auxiliary organic pigment capable of making the polarity of the impressed electric charge distinct. The Okuno patent provides an improvement over the conventional use of asphalt or rosin by providing a pigment coated with a specifically defined synthetic resin copolymer.

A further conventional liquid developer is disclosed in US. Pat. No. 3,438,904 to Wagner, the liquid developer or toner being one which includes an evaporative liquid hydrocarbon carrier, a minor amount of a finely divided opaque insoluble pigment dispersed in a liquid carrier and a manganese salt of an organic acid to improve the quality of the image produced by the pigment particles. The Wagner patent further indicates that the pigment or particles introduced into the liquid developer or toner may be introduced with or without a coating of a natural or a synthetic resin or oil such as asphalt.

While both the above patents describe the possible addition of asphalt to a liquid developer, each patent is directed to a positive charging of coloring agent particles by controlling the polarity of the same with the addition of a charge controlling agent in the liquid carrier.

In accordance with the present invention, however, in

a manner totally contrary to these procedures, it has been discovered that through the use of a specifically identified extract of asphalt, it is possible to charge the polarity of coloring agent particles negatively.

Accordingly, it is a principal object of the present invention to provide a liquid developer which eliminates the foregoing deficiencies and disadvantages of previously proposed developers.

It is a further object of the present invention to provide a liquid developer for electrophotography which liquid developer, in a manner unlike previously proposed developers, is capable of negatively charging particles of coloring material thereby giving a reverse visible image in the case of a negative electrostatic latent image or a non-reverse image in the case of a positive electrostatic latent image.

It is yet a further object of the present invention to provide a liquid developer for electrophotography wherein the liquid developer comprises particles of an image forming material dispersed in a composite dispersion medium comprising aliquid carrier of high electric resistance and an asphalt extract which is at least substantially completely dissolvable in the liquid carrier and which is obtained by extracting an asphalt with a saturated hydrocarbon liquid.

Still further objects and advantages of the present invention will become more apparent from the following more detailed description of the present invention.

In accordance with the present invention, a liquid developer is providedfor converting electrostatic latent images to visible images which consists essentially of a composite dispersion medium of a liquid of high. electric resistance containing, dissolved therein, an extract of an asphalt with a saturated hydrocarbon liquid, the liquid of high electric resistance being capable of at least substantially dissolving the extract therein, and dispersed in the composite dispersion medium, particles of at least one image forming material selected from dyestuffs, pigments and powdery metals.

Asphalt is known to be a complicated colloidal mixture comprising a disperse phase composed of high molecular weight aromatic compounds, a dispersion medium composed of saturated aliphatic compounds, and groups of low molecular weight aromatic compounds and of naphthenic and aromatic compounds or saturated aliphatic compounds, which act as a peptizer dispersing the disperse phase into the dispersing medium. When such asphalt per se is dispersed into a liquid of a high electric resistance, the dispersed asphalt particles, as is well known in the art, are positively charged. Unexpectedly, it has been found that when an extract of an asphalt with a saturated hydrocarbon liquid is dissolved in a liquid of high electric resistance capable of substantially dissolving the extract therein, the resulting composite dispersion medium charges particles of pigments and the like negatively when they are dispersed in the composite dispersion medium.

As indicated above, asphalt is a complicated colloidal mixture, the properties of the extract of which can vary considerably depending on the type of asphalt used, the type of extraction solvent used and the extraction conditions, the exact nature of the extract being difficult to specifically determine. The following discussion, however, will attempt to define the type of asphalt extract which is applicably employed in accordance with the present invention.

According to the composition analysis of asphalt by the US. Mining Bureau, asphalt can be analyzed as follows:

preferred, the present invention is also applicable to natural asphalt such as, for example, gilsonite, although the extraction of natural asphalt is not preferred since the extraction of the same produces less resin content and oil content.

The straight asphalts and blown asphalts which are capable of extraction in accordance with the present invention can be any of these conventional asphalts regardless of penetration. It is preferred, however, that the penetration of the asphalt, based upon test method 115 K2530-l960, be about 0 150 in the case of a straight asphalt and about 5 in the case of a blown asphalt. Again, however, these penetration values are preferred and the present invention is applicable to the conventionally employed petroleum asphalt, straight asphalt and blown asphalt regardless of specific physical properties.

As indicated above, the extract of asphalt is obtained in accordance with the present invention by extracting the asphalt with a saturated hydrocarbon liquid. The saturated hydrocarbon liquids suitably employed in accordance with the present invention can be selected from straight chain aliphatic hydrocarbons, branched chain aliphatic hydrocarbons and alicyclic hydrocarbons. Specific examples of applicable straight chain ali Insoluble carbene,carhoid 'ontcnt lnsoluble Benzene content treatment Soluble Solvent content removal Asphalt n pentane treatment asphaltene Adsorption methanollt can be seen from the above that the composition of asphalt can be broadly classified into the following three components:

a. Asphaltene (n-pentane insoluble content) b. resin content (n-pentane soluble content) c. oil content (n-pentane soluble content). The extract of asphalt with the saturated hydrocarbon in accordance with the present invention is believed to be substantially composed of the above-noted resin content and oil content. It should be recognized, however, that the exact composition of the extract is not critical to the present invention, the only requirement being that the extract is at least substantially dissolvable, preferably dissolvable to an extent of at least 98%, most preferably completely dissolvable in the liquid carrier which is employed in the liquid developer.

The asphalt which is extracted in accordance with. the present invention can be any conventional asphalt, the preferred asphalt being petroleum asphalts, straight asphalts and blown asphalts. While these asphalts are phatic hydrocarbons include those with 5 to 18 carbon atoms, specifically straight chain aliphatic hydrocarbons have 5 to 16 carbon atoms. Representative compounds of the above type include, for example, npentane, n-hexane, n-heptane, n-octane, n-decane, nundecane, n-dodecane,' n-tridecane, n-hexadecane, noctadecane, etc.

' Suitable branched chain aliphatic hydrocarbons include those of from 5 to 15 carbon atoms, preferably tion are those which contain from 5 to 8 carbon atoms, preferably 5 or 6 carbon atoms. Representative examples include'cyclopentane, cyclohexane, methylcyclohexane.

In preparing the extract of asphalt by extracting one of the aforementioned alphalts with one of the abovenoted saturated hydrocarbon liquids, it is merely necessary to contact the asphalt with the suitable saturated hydrocarbon liquid without taking any necessary precautions. In this respect, the temperature of the extraction is in no way critical to the present invention and the extraction is generally carried out at ambient temperature. Certainly slight deviations from ambient temperature, through cooling or heating of the extraction medium, does not in any way effect the extraction or extract of asphalt which is obtained. Similarly, the ratio between the asphalt and extraction solvent is in no way critical to the present invention, although in general practice, 5 to 1000 parts of extraction solvent are employed for 1 part of asphalt, the parts being by weight.

In preparing the liquid developer for electrophotography of the present invention, the extract of asphalt is at least substantially dissolved, preferably completely dissolved in a carrier liquid of high resistance. The carrier liquid of the present invention is a liquid of a dielectric constant of not more than 3, and a resistance of not less than ohm-centimeter. Specific types of carrier liquids applicably employed include for example, straight chain aliphatic hydrocarbons, branch chain aliphatic hydrocarbons, halogenated hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, mixed hydrocarbon liquids and other conventional carrier liquids including, for example, siloxanes such as dimethyl polysiloxane.

The straight chain aliphatic hydrocarbons which can be employed as the carrier liquid are generally those having from 5 to 12 carbon atoms, preferably 9 to 12 carbon atoms. Representative examples include for instance, n-pentane, n-hexane, n-heptane, n-octane, nnonane and n-decane.

The branch chain aliphatic hydrocarbons useful as the carrier liquid in accordance with the present invention are generally those having from 5 to 12 carbon atoms, preferably 9 to 12 carbon atoms. Representative examples include isohexane, isooctane,- 2,2- dimethylbutane, isopentane and lsopar E (C lsopar G" (C Isopar H (C Isopar L m-12),ls0par M (C m), Shellsol 71 (product of Shell Petroleum Company) and the like which are commercially available as isoparaffin type hydrocarbon mixtures. The halogenated hydrocarbons suitably employed in accordance with the present invention generally include those having up to about 6 carbon atoms. Representative examples include carbon tetrachloride, fluorotrichloromethane (Freon -11) and trifluorotrichloroethane (Freon 13).

The alicyclic hydrocarbons useful in accordance with the present invention are generally those having from 5 to 8 carbon atoms, preferably 5 to 7 carbon atoms,

including cyclohexane, methylcyclohexane, and cyclopentane.

The aromatic hydrocarbons useful in accordance with the, present invention'are those which generally have from 6 to 9 carbon atoms, preferably 6 to 8 car bon atoms including, for example, benzene, toluene and xylene. e

As mixed hydrocarbon liquids suitable for use in accordance with the present invention, reference can bemade for example, to kerosene (petroleum distillate with a boiling point of not more than 320C); Whitesol (tradename of Kyodo Oil Co., Ltd), petroleum naphtha (ligroin, benzene), solvent naphtha. Whitesol is a mixed hydrocarbon liquid characterized by the following physical properties:

Composition analysis aromatic content 7n In accordance with the present invention the particular carrier liquid which is selected in preparation of the liquid developer can be freely selected from the above types, it being important only that the carrier liquid which is employed be one which has a solubility for the extract which is equal to or higher than that of the extractionsolvent. Such solubility, of course, can be easily determined with a minimum amount of experimentation..For example when a straight or branched aliphatic hydrocarbon is used as the extractant, the electrically insulating liquid used may not only be the above-recited straight and branched aliphatic hydrocarbons, but also the alicyclic hydrocarbons and halogenated hydrocarbons. When an alicyclic hydrocarbon is used as the extractant, alicyclic hydrocarbons and halogenated hydrocarbons are preferably used as the electrically insulating liquid, but use of straight or branched aliphatic hydrocarbons is not preferred. When aromatic hydrocarbons such as benzene are used as the extractant, a considerable portion of the extract remains undissolved in a conventional, electrically insulating dispersion medium and the tendency that the dispersed pigment particles are positively charged increases. Accordingly, it is not preferred to use aromatic hydrocarbons as the extractant for asphalt.

In the liquid developer of this invention, the effective concentration of the asphalt extract in the composite dispersion medium varies depending on the type of coloring material to be dispersed, the extraction medium used for the extraction of the asphalt and the liquid medium of high electric resistance. However, generally speaking, if more than 0.1 g of the asphalt extract is present in 1 liter of the dispersion medium, it is possible to effectively negatively charge the dispersed particles to the image forming material. When too great an amount of the asphalt extract is present in the dispersion medium, no particular advantage is obtained by the increase of the amount of the asphalt extract and a tendency appears that the distinctness of the copy is damaged by the presence of the excessive extract. Accordingly, in the present invention, the asphalt extract is generally used in an amount of 0.1 to 30 g per liter, preferably 0.5 to 3 g per liter of the composite dispersion medium.

There is no critical limitation on conditions for preabove-mentioned asphalt extract is mixed into a liquid I of high electric resistance so that the above-mentioned conditions will be satisfied, a composite dispersion mep 7 ti iiim can he iasi l y obitine'i. Whn'ilie' eiiaaiis used" without isolation of soluble components from the ext r actant, the resulting composite dispersion medium also contains the saturated hydrocarbon liquid used as As pigments commercially available inorganic and Y organic pigments may be used.'For instance, in organic pigments such as carbon black, cadmium selenide, cadmium sulfide, black iron oxide, Ultramarine and chrome yellow, and organic pigments such as Lithol the extractant.

The coloring material used in this invention as an Red R Phthalocyanme Blue}; image forming material is generally selected from dye- 74160) fl g ggg hik S- stuffs, pigments, powdery metals and mixtures thereof. 2 F g g an om ast It is essential that these image forming materials should Red G l can l t be free of asphalt especially high molecular weight aro- A5 Powdery meta S Powders o. metals Such as a matic components contained in asphalt because these m and copper "F used high molecular weight aromatic components contained h present mvmtmn these i i f' in asphalttend to decrease the above-mentioned spei q f g z g be g mgly or zi cific surface-activating property of the composite dis-, Pamc t ese .yestu plgm ems an R ery persion medium of this invention g metals, whendlspersed in the composite d spersion me- Any conventional dyestuff insoluble in the composite of mvennon are equally neganvely, .charged' dispersion medium canbe used as the image forming regarqless of whe.ther' are.charged q l or material. Examples of such dyestuffs areas follows: "Q WF X dlspersed m ordmar)" elecmcauy Wm A l l .'fl9l .2 (l Substantive Dyes: g The image forming material such as recited above is A {1 23338; usually used in an amount of 0.5 to 5 g per liter, prefer- Direct Deep Black RW (c. I. 30245 ably 0.1 to 1.5 g per liter of the composite dispersion Benzamin Black VS (C. I. 30015) di P (2) Acid Dyes: I I v M.--

Rl'illiagt Scalrlel; 3R c. I. 16255) A d ta l d hereinabove, the present invention a ose enga e. (C. I. 45440) Brilliant Bl pcp (C L 42090) novel effect is prov ded in that part cles of an image i l Black 105 (c. i. 10470) jgljmslllg material such as dyestuffs, pigments and pow- (3) Creen 68 (C. 44090) d t l be negat vely charged eql lally by the Crystal Violet- ('C. I. 42555 use of a specific composite dispersion medium formed Methylene Blue B Cone (C. l. 520l5) ted Janus Brown (a L 33505) by dissolv ng an extract of an asphalt with a satura (4) Mordant Dyes and Acid Mordant hydrocarbon liquid into a liquid medium of a high elec- Dyes: ch Brown RH (C I 13250) tric resistance. Further, the charge of the dlspersedpar Alizarin Yellow c. l. l4025 ticles is kept negative by theabove-mentloned peculiar Chrome Black PB (c. 1. 14645) site dis rsion Chrome Cyanine R c. I. 43820) surface'actwatmg. property q l compo fi. Eriochrome Azurol G c. 1. 43855) medium. Accordingly, the liquid developer 0 t 15 n- (5) Dyes 1 vention is advantageous in that the properties of the liq- Sulfur Black 8 (C. l. 53l85) h (6) Vat D es.- uid developer are not appreciably changed with t e i Dir"! Black RB 65230) lapse of time and it is stable during long-time usage or (7) Dlsperse Dyes:

Disperse Fast Violet 5R (c. I. 61100) Storage- I i p a Blue FFB (C- I- 62035) From the data shown in Table 1 below it is apparent (8) ilg o l u blz gyzg D H365) that is is important in this invention that the asphalt exi Blue B Base (6 44045 tract is substantially dissolved in a dispersion medium tiilii i' 'ia'itl zi ii llis lsw s. le sees? TABLE 1 Charge Polarity of Particles Attained when They Are Dispersed in Composite Dispersion Medium Comprising Liquid tfi shfilss r 3????51'1? ai .d A igh ltlixtrast I Charge Polarity of Image F H Image Forming Material Dispersion Jjirniing Material Liquid of Liquid Used for Extraction of Asphalt H h Kind of Dyestuff, Pig- Electric n-Heptane lsopar H Cyclo Benzene ment of Powdery Metal Resistance Hexane Carbon Black lsopar H Carbon Black Carbon Tetrachloride Phthalocyanine Blue lsopar H Phthalocyanine Blue n-Heptane Phthalocyanine Blue Whitesol Victoria Blue Base n-Heptane V Lithol Red Cyclohexane Lithol Red N-Hexane Crystal Violet n-Hexane Crystal Violet Kerosene Rose Bengale lsoper H Aluminum Powder n-pentane Whitesol Copper Powder With reference to the solvents indicated in Table l, the degree of the ability to dissolve asphalt is in the following order:

n-heptane n-pentane n-hexane lsopar H Cyclohexane Kerosene Benzene Carbon tetrachloride Whitesol v EXAMPLE 1 parts of straight asphalt of a penetration of 80-100 (product of Maruzen Oil Co.) are added and dissolved into 100 parts of n-pentane, and insoluble matters are removed by filtration. Then n-pentane is evaporated and components of the asphalt soluble in n-pentane are extracted. With-use of the resulting asphalt extract, a liquid developer is prepared in the following manner.

2 g of the extract of the straight asphalt with npentane are completely dissolved in 1 liter of n-hexane which is an electrically insulating liquid.'Then, 0.1 g of Crystal Violet is added to the solution and dispersed therein over a period of 5 minutes under ultrasonic vibration. V

Electrode plates are put into the so formed liquid and a direct electric current of 1000 V is impressed for l minute. Then, the electrode plates are taken out, and the state of the image forming material particles appended to the electrode plate surface isobserved. As a result, it is seen that the particles are completely neg atively charged l00%). This liquid developer converts a negatively charged, electrostatic liquid image on a polyvinyl carbazole photosensitive paper to a reverse visible image of a violet color and converts a positively charged electrostatic latent image on a polyvinyl carbazole photosensitive paper to a non-reverse visible image of a violet color. I

When these printed sheets are contacted with steam or heated at 120C. for 2-3 seconds, very distinct bluish violet images of excellent fixation are obtained.

. EXAMPLE 2 10 g of straight asphaltof a penetration of 80-100 (product of Maruzen Oil Co.) are added and dissolved in 100 g of lsopar H (isoparalfin type hydrocarbon; product of Esso Standard Oil Co.), and insoluble matters are removed by filtration. With use of the resulting brown extraction liquid, a liquid developer is prepared in the following manner.

Separately, a negatively charged zinc oxide photosensitive paper is exposed to light together with a negative original to form a negative latent image. When the so treated photosensitive paper is dipped in the above liquid developer, a positive black image is obtained.

EXAMPLE 3 10 parts of blown asphalt of a penetration of 10-20 (product of Nippon Oil Co.) are added and dissolved into 100 parts of n-pentane, and insoluble matters are removed by filtration. Then, n-pentane is evaporated and componentsof the blown asphalt soluble in nheptane are extracted. With use of the so prepared extract, a liquid developer is prepared in the following manner.

1 g of the above extract of the blown asphalt with nheptane is completely dissolved in 1 liter of kerosene which is anelectrically insulating liquid, and 0.3 g of Nigrosine Base, which is an oil-soluble dye, is added to the solution and dispersed therein over a period of 3 minutes by means of a homomixerto forma liquid developer.

Separately, a negatively charged zinc oxide photosensitive paper is, exposed to light together with a positive original. The, it is dipped into the above liquid developer. Thus a negative black image is obtained.

EXAMPLE 4 10 g of ablown asphalt of a penetration of 30-40 (product of Nippon Oil Co.) are added to 100 g of nhexane and dissolved therein. Insoluble matters are removed by filtration With the use of the resulting extraction liquid of a brown color, a liquid developer is prepared in the following manner. I

10 g of. the above extract of the blown asphalt with n-hexane are diluted with 1 liter of n-hexane as the electrically insulating liquid and 0.6 g of Lithol Red, which is an organic pigment, is added to the dilution and dispersed therein over a period of 3 minutes under ultrasonic vibration.

v When theso formed liquid developer is used for the development of a negative electrostatic latent image on a negatively charged zinc oxide photosensitive paper, a reverse image of ared color is obtained.

In case a liquid developer free of the above extract of the blown asphalt with n-hexane is used, the polarity of the pigment particles is positive.

EXAMPLE 5 10 parts of straight asphalt of a penetration of -100 (Product of Maruzen Oil Co.) are added and dissolved into parts of n-pentane, and insoluble matters are separated by filtration. Then, n-pentane is evaporated and components of the straight asphalt soluble in n-pe'ntane are extracted. With use of the so prepared extract, a liquid developer is prepared in the following manner.

4 g of the above extract of the straight asphalt with n-pentane are completely dissolved in 1 liter of npentane as electrically insulating liquor, and 1 g of aluminum powder as metallic image forming material is added to the solution'and dispersed therein over a period of 5 minutes under ultrasonic vibration to form a liquid developer.

Separately, a negatively charged zinc oxide photosensitive paper is exposed to light together with a negative original. When the so treated photosensitive paper 1 1 is dipped into the above liquid developer, a positive image of a silver color is obtained.

When a liquid developer free of the above extract of the straight asphalt with n-pentane is used, the polarity of the aluminum particles is positive.

EXAMPLE 6 I n-heptane are completely dissolved in l liter of nheptane, which is an electrically insulating liquid, and

0.3 g of Victoria Blue Base, which is an oil-soluble dye, is added to the solution and dispersed therein over a pe riod of 3 minutes by means of a homomixer to form a liquid developer.

When the so prepared liquid developer is used for the development of a negative electrostatic latent image of a negatively charged zinc oxide photosensitive paper, a reverse image of a brown color is obtained.

When a liquid developer free of the above extract of the straight asphalt with n-pentane is used, the polarity of the coloring material particles ispositive.

EXAMPLE 7 10 parts of blown asphalt of a penetration of l-20 (product of Nippon Oil Co.) are added and dissolved into 100 parts of n-pentane and insoluble matters are removed by filtration. Then, n-pentane is evaporated, and components of the blown asphalt soluble in npentane are extracted. With use of the obtained extract, a liquid developer is prepared in the following manner.

15 g of the above blown asphalt extract is completely ing a negative electrostatic latent image, a reverse image of a red color is obtained.

EXAMPLE 9 parts of straight asphalt of a penetration of 120-150 (product of Maruzen Oil Co.) are added and dissolved into 100 parts of n-pentane, and insoluble matters are removed by filtration. Then, npentane is evaporated and components of the straight asphalt soluble in n-pentane are extracted. With use of the resulting extract, a liquid developer is prepared in thefollowing manner.

1 g of the above extract of the straight asphalt with n-pentane-is completely dissolved in 1 liter of Shellsol 71 (isoparaffin type hydrocarbon; product of Shell Petroleum Co.) which is an electrically insulating liquid, and 0.1 g of Phthalocyanine Blue which is an organic pigment is added to the solution and dispersed therein over a period of 3 minutes by means of a homomixer to forma liquid developer.

When the so formed liquid developer is applied to a zinc oxide photosensitive paper having a negative electrostatic latent image, a reverse image of a blue color dissolved into 1 liter of carbon tetrachloride, which is I an electrically insulating liquid. Then, 0.15 g of carbon black (F W-ZV) is added to the solution and dispersed therein over a period of 3 minutes under ultrasonic vibration to form a liquid developer.

When this liquid developer is applied to a zinc oxide photosensitive paper having a negative electrostatic latent image, a reverse image of a black color is obtained.

EXAMPLE 8 l0 parts of straight asphalt of a penetration of 80l00 (product of Maruzen Oil Co.) are added and dissolved into 100 parts of cyclohexane, and insoluble matters are removed by filtration. With use of the resulting extraction liquid of a brown color, a liquid developer is prepared in the following manner.

When the so formed liquid developer is used for the development ola zinc oxide photosensitive paperhavis obtained.

EXAMPLE l0 10 parts of blown asphalt of a penetration of 10-20 (product of Nippon Oil Co.) are added and dissolved into l00 parts of n-pentane, and insoluble matters are removed byfiltration. Then, n-pentane is evaporated and components of the blown asphalt soluble in npentane' are extracted. With use of the so recovered extract, a liquid developer is prepared in the following manner.

1.5 g of the above extract of the blown asphalt with n-pentane is completely dissolved in 1 liter of Whitesol (product of Kyodo Oil Co.), which is an electrically insulating liquid and 0.1 g of Phthalocyanine Blue is added to the solution and dispersed therein over a period of 3 minutes by means of a homomixer to form a liquid developer.

When the so formed liquid developer is used for the development of an electrostatic latent image on a negatively charged zinc oxide photosensitive paper, a reverse image of a blue color is obtained.

EXAMPLE i1 ered extract, a liquid developer is prepared in the following manner. i

2 g of the above extract' of the straight asphalt with n-pentane are completely dissolved in 1 liter of Isopar G (product of Esso Standard Oil Co.), which is an electrically insulating liquid, and 0.6 gof Rose Bengale, which is an acid dye, is added to the solution and dispersed therein over a period of -3 minutes by means of r a homomixer.

When the so prepared liquid developer is applied to the development of an electrostatic latent image on a negatively charged zinc'oxide photosensitive paper, a reverse image of'a faintly red color is obtained.

soluble content was removed by filtration. Then, noctane was evaporated and the component of the straight asphalt soluble in n-octane was extracted. A

liquid developer was prepared by the use of the extract obtained by the method described below.

2 g of n-octane extract of straight asphalt were completely dissolved in 1 liter of Shellsol 71 (Shell Petroleum Co., Ltd. made isoparaffin type hydrocarbon solvent) as an insultaing liquid and 0.2 g of carbon black (Degussa Company made FW-ZV) as a coloring agent was added thereto and dispersed using ultrasonic wave during a period of minutes. In this way a liquid developer was prepared.

Positively charged polyvinyl carbazole photosensitive paper was exposed to light with the positive original and soaked in the above liquid developer and a positive black image was obtained.

EXAMPLE l3 parts of straight asphalt (with a penetration of 40-60) produced by Daikyo Oil Co., Ltd. were mixed with and dissolved in 100 parts of n-dodecane and the insoluble content was removed by filtration. Then, ndodecane was evaporated and the component of the straight asphalt soluble in n-dodecane was extracted. A liquid developer was preparedby the use of the extract obtained by the following method.

2 g of the n-dodecanc extract of the straight asphalt were completely dissolved in 1 liter of lsopar G (Esso Standard Oil Co., Ltd. made isoparaffin hydrocarbon) as an insulating liquid and 0.2 g of carbon black (Degussa Company made FW-2V) as a coloring agent was added thereto and dispersed using ultrasonic wave during a period of 5 minutes whereby a liquid developer was prepared.

Negatively charged zinc oxide photosensitive paper was exposed to light with the negative original and soaked in the above liquid developer and a positive black image was obtained.

EXAMPLE l4 10 parts of straight asphalt (with a penetration of 80l00) produced by Daikyo Oil Co., Ltd. were mixed with and dissolved in 100 parts of n-decane and the insoluble content was removed by filtration. Then, ndecane was evaporated and the component of the straight asphalt soluble in n-decane was extracted. A-

liquid developerwas prepared by the use of the extract obtained by the following method.

2 g of straight asphalt extract were completely dissolved in 1 liter of lsopar G as an insulating liquid and 0.7 g of Spirit Black (C. I. 50415) was added thereto and dispersed using ultrasonic wave during a period of 5 minutes and a liquid developer was prepared.

A positive electrostatic latent image was formed by electrostatic transfer on recording paper obtained by coating Aron SlOOl (Toa Gosei Chemical Industry Co., Ltd. made acrylic ester resin) to a thickness of 10 ,u. on an electroconductive base plate, followed by drying, and soaking in the above liquid'developer whereby a highly contrasted black image was obtained.

EXAMPLE 15 10 parts of straight asphalt (with a penetration of 120-150) produced by Daikyo Oil Co., Ltd. were mixed with and dissolved in parts of n-hexadecane and the insoluble content was removed by filtration. Then, n-hexadecane was evaporated and the nhexadecane soluble component of the straight asphalt was extracted.

A liquid developer was prepared by the use of the extract obtained by the following method.

.2 g of n-hexadecane extract of the straight asphalt were completely dissolved in 1 liter of lsopar G as an insulating liquid and l g of metal zinc powder was added thereto and dispersed using ultrasonic wave during a period of 5 minutes and a liquid developer was prepared.

Negatively charged zinc oxide photosensitive paper was exposed to light with the negative original and soaked in the above liquid developer and a positive silver colored image was obtained. The reproduction was somewhat poor in fixation, but it was fixed by being soaked in vinyl acetate resin 2% alcohol solution, followed by drying.

EXAMPLE l6 Dia Black SH (Mitsubishi Chemical Industries, Ltd.

made carbon black)--30 g Cyanine Blue GNPS (Kabushiki Kaisha Ozeki-Sha made copperphthalocyanine)-3.5 g

lsopar H Esso Standard Oil Co., Ltd. made isoparaffin hydrocarb0n)-l20 g LR-472 (Dai Nippon lnk & Chemicals Inc. made modified acryl resin)-42 g Dibutylphthalate-34 g Asphalt extract [Blown asphalt made by Daikyo Oil Co., Ltd. (with a penetration of 5-10) was extracted with n-tridecanel-2 g The above mixtures was dispersed by means of a homomixer for 30 minutes and 8 ml of the concentrated solution obtained were dispersed in 1 liter of lsopar L (Esso Standard Oil Co., Ltd'made isoparaffin hydrocarbon) and asa result a liquid developer was prepared.

Electrode plates were inserted into the developer solution and a direct current 1300 V was applied for 30 seconds and thereafter the plates were taken out of the solution and the state of the deposited image forming material particles on the surface of electrode plates was obtained. As a result, it was found that the image forming material had 100% negative electric charge. A re versed black image with excellent fixation was obtained when the negatively charged electrostatic latent image on zinc oxide photosensitive paper was developed by the use of the above liquid developer.

EXAMPLE 1 7 10 parts of blown asphalt (with a penetration of 20-30) produced by Daikyo Oil Co., Ltd. were mixed with and dissolved'in 100 parts of n-undecane and the insoluble content was removed by filtration. Then, nundecane was evaporated and the n-undecane soluble component of the blown asphalt was extracted. A liquid developer was prepared by the use of the extract so obtained by the following method.

2g of blown asphalt extract were completely dissolved in 1 liter of lsopar L (Esso Standard Oil Co.,

Ltd. made isoparaffin hydrocarbon solvent) as an insulating liquid and 1.0 g of Brilliant Green (C. I. 42040) was added thereto and dispersed using ultrasonic waves for 5 minutes, thereby preparing a liquid developer.

The particle polarity was negative and the particles were adsorbed to the positive latent image thereby forming a green image.

EXAMPLE l8 thereby preparing a liquid developer.

The liquid developer obtained had negatve electric charge and a reversed red image with excellent fixation being obtained when the negatively charged electrostatic latent image on zinc oxide photosensitive paper was developed by the use of this liquid developer.

EXAMPLE l9 10 parts of straight asphalt (with a penetration of -40) made by Mitsubishi Oil Co., Ltd. were mixed with and dissolved in 100 parts of iso-octane and the insoluble content was removed by filtration. Then, isooctane was evaporated and the iso-octane soluble component of the straight asphalt was extracted.

A liquid developer was prepared by the use of the extract by the following method.

2 g of iso-octane extract of straight asphalt were completely dissolved in 1 liter of lopar L as an insulating liquid and 0.2 g of carbon black (Mitsubishi Chemical lndustries, Ltd. made Dia Black SH) was added thereto and dispersed using ultrasonic waves for 5 minutes, thereby preparing a liquid developer.

A reversed black image was obtained when the negatively charged electrostatic latent image on polyvinyl carbazole photosensitive paper was developed by the use of this liquid developer, whereas a positive image was obtained when the positively charged electrostatic latent image was developed.

EXAMPLE 20 SLC-5l2)5g lsohcxanc extractlS g Shellsol 7l (product of Shell Petroleum Co.,

Ltd.)- -200 ml A concentrated toner was prepared by dispersing the above mixture using ultrasonic waves for 1 minute.

50 ml of concentrated toner were stirred and diluted with Shellsol 71 using a glass stick, thereby preparing a liquid developer.

When negatively charged zinc oxide photosensitive 5 paper was exposed to light withthe negative original and soaked in the above liquid developer and a positive black image was obtained.

EXAMPLE 21 10 parts of straight asphalt (with a penetration of 80-100) made by Mitsubishi Oil Co., Ltd. were mixed with and dissolved in 100 parts of 2,2-dimethylbutane and the insoluble content was removedby filtration. Then, 2,2-dimethylbutane was evaporated and the 2,2-

15 dimethylbutane soluble component of the straight asphalt was extracted.

A liquid developer was prepared by the use of the extract by the following method.

Colortex Blue No. 150 (Sanyo Shikiso Kabushiki Kaisha made resin finishing pigment)-5 g 2,2-dimethylbutane extract-15 g Whitesol (Kyodo Oil Co., Ltd. made aliphatic, aromatic mixed solvent)200 ml A concentrated toner wasprepared by dispersingthe 25 above mixture using ultrasonic waves for 1 minute.

ml of the concentrated toner were stirred and diluted with 1 liter of Whitesol using a glass stick. thereby preparing a liquid developer.

When positively charged polyvinyl carbazole photosensitive paper was exposed to light with the positive original and soaked in the above liquid developer a positive blue image was obtained.

EXAMPLE 22 above mixture using ultrasonic waves for one minute. I

50 ml of concentrated toner were stirred and diluted with 1 liter of Whitesol using a glass stick, thereby preparing a liquid developer.

The particle polarity in this liquid developer was charged negatively.

Examples 23-30:

The same procedure as in Example 12 was repeated by the use of the coloring agent, asphalt, extraction solvent and carrier tabulated in the following Table 2.

Comparative Examples: Sample 1 0.5 g of Dia Black SH (Mitsubishi Chemical lndustries, Ltd. made carbon black), 2.5 g of straight asphalt (product of Maruzen 'Oil Co., Ltds.; penetration: 80400) and cc of lsopar H (Esso Standard Oil Co, Ltd. made isoparaffin hydrocarbonsolvent) were mixed and dispersed by ultrasonic waves for 5 minutes, thereby forming concentrated solution and subsequent TABLE 2 Exam- Coloring Agent Type of Asphalt Extraction Carrier Particle Polarity l Solvent of liquid developer 23 Methyl Violet Red 0.5g Mitsubishi Oil Co., lsopar G Whitesol negative Ltd. (Esso Standard (Kyodo Oil Extract 2g Blown asphalt Oil Co. Ltd. Co., Ltd. Carrier l liter (penetration: lsoparaffin -40) hydrocarbon) (C.l. 45160) Mitsubishi Oil Co.. lsopar H 24 Rhodaniine 6 0.5g Ltd. Blown (Esso Standard lsopar H d Extract 2g Asphalt (penetration Oil Co., Ltd. Carrier I 1 liter l()-20) isoparalfin hydrocarbon) 25 Carbon Black 0.l5g Maruzen Oil Co., Methylcyclo- Xylene do.

Cyanine Blue GNPS 002g Ltd. Straight hexane asphalt Piccolastic A-75 0.0l5g (penetration:

(Nippon Petro.K.K.) Extract 0.4g Carrier l liter (CI. 45380) 26 Eosin 0.5g Maruzen Oil Co., Cyclopentane Cyclopentane do.

Extract 2g Ltd. Straight Carrier 1 liter asphalt (penetration: 40-60) 27 Lithol Red 0.8g Nippon Oil Co., Ltd. n-hexadecane fluorodo.

Extract lg Straight asphalt trichloro Carrier '1 liter (penetration: 20-40) methane 28 Aluminum Nippon n-decane cyclohexane do.

Oil Co..

Ltd. powder lg Straight asphalt Extract 5g (penetration: Carrier l liter 20-40) 29 Carbon Black 0.1g Nippon Oil Co., Ltd. [so-hexane lso-hexane do.

.l-534(alkyd) Straight asphalt (Dai Nippon lnk (penetration:

Chemical Co., 60-80) Ltd.) 0.3g Extract 0.3g Carrier 1 liter 30 Carbon Black 0.2g Nippon Oil Co., Ltd. n-pentane dimethylpoly- Extract r 2g Blown asphalt siloxane do. Carrier 1 liter (penetration:

Results:

to cooling, 0.8 ml of the concentrated solution was diluted with 100 cc of lsopar H to prepare a liquid developer.

Sample 2 10 g of straight asphalt (product of Maruzen Oil Co., Ltd.; penetrationzSO-IOO) were dissolved in 100 cc of n-pentane andthe insoluble content was removed by filtration. Some 150 mg of this extract were added to the liquid developer of Sample 1 and completely dissolved to prepare a liquid developer.

Sample 3 0.5 g of Dia Black SH, 1.25 g of n-pentaneextract substance obtained by Sample 2 and 10 cc of lsopar H were mixed and dispersed by ultrasonic waves for one minute, thereby forming. a concentrated solution, and subsequent to cooling, 0.8 ml of concentrated solution was diluted with 100 cc of lsopar H, thereby preparing a liquid developer. v

The particle polarity of the above samples was measured and the results are tabulated in Table-3.

TABLE 3 Samples 2- v, 3

Comparative Comparative The lnstant Example Example lnvention Particle Positive Positive Negative Polarity 1. The particle polarity of the liquid developer (Sample l) in which carbon black was coated with asphalt indicated to be positive.

2. The particle polarity of the liquid developer (Sample 2) where the n-pentane extract of asphalt was dissolved in the carrier solution and added to the liquid developer (Sample 1) in which carbon black was coated with asphalt indicated to be positive.

It should be evident from the above that the liquid" developer for 'electrophotography in accordance with the present invention produces results unlike previously developed liquid developers through the combination of particles of an image forming material in a composite dispersion medium comprising a liquid carrier of high electric resistance and an asphalt extract obtained by extracting an asphalt with a saturated hydrocarbon liquid, the extract being at least substantially completely dissolvable in the liquid carrier.

While the present invention has been described primarily by reference to the foregoing exemplification, it should be understood that the present invention is in no way to be deemed as limited thereto, but rather must be construed as broadly as all or any equivalents thereof.

We claim:

1. A liquid developer for developing electrostatic latent images which comprises a. a composite dispersion medium consisting essentially of i. a liquid carrier of a dielectric constant of not more than 3 and aresistance of not less than ohm-centimeter, and

ii. an extract of asphalt which is at least substantially completely dissolvable in said liquid carrier, said extract of asphalt being obtained by extracting an asphalt selected from petroleum asphalts, straight asphalts, blown asphalts and natural asphalts with a saturated hydrocarbon liquid from straight chain aliphatic hydrocarbons of from 5 to 18 carbon atoms, branched chain aliphatic hydrocarbons of from 5 to carbon atomsand alicyclic hydrocarbons of from 5 to 8 carbon atoms, and

b. an image-forming material dispersed in said composite dispersion medium selected from dyestuffs, pigments and powdery metals, said extract of asphalt being present in an amount of0.l to 30 grams per liter of said composite dispersion medium, the particles of said image-forming material being negatively charged in said composite dispersion medium.

2. The liquid developer of claim 1 wherein said straight chain aliphatic hydrocarbon is selected from n-pentane, n-hexane, n-heptane, n-octane, n-decane, n-undecane, n-dodecane, n-tridecane, n-hexadecane and n-octadecane.

3. The liquid developer of claim 1 wherein said branched aliphatic hydrocarbon is selected from isopentane, isohexane, 2,2-dimethylbutane, isooctane, and C -C isoparaffin mixtures (boiling point 176.7 186C), C,,C,- paraffin mixtures (boiling point 159 176.7C.) and lsopar R.

4. The liquid developer of claim I wherein said alicyclic hydrocarbon is selected from cyclopentane, cyclohexane and methylcyclohexane.

5. The liquid developer of claim 1 wherein said straight chain aliphatic hydrocarbon contains from 5 to 16 carbon atoms, said branched chain aliphatic hydro-,

carbon contains from 5 to 12 carbon atoms and saidalicyclic hydrocarbon contains 5 or 6 carbon atoms.

6. The liquid developer of claim I wherein said liquid carrier is selected from straight chain aliphatic hydrocarbons, branched chain aliphatic hydrocarbons, halogenated hydrocarbons, alicyclic hydrocarbons, are-- matic hydrocarbons, mixed hydrocarbon liquids and dimethyl polysiloxane.

7. The liquid developer of claim 1 wherein said extract of asphalt is present in an amount of 0.5 3 grams 5 per liter of said composite dispersion medium.

8. The liquid developer of claim 1 wherein said image forming material is present in an amount of 0.05 to 5 grams per liter of said composite dispersion medium.

9. The liquid developer of claim 8 wherein said image 10 forming material is present in an amount of from 0.1 to

1.5 grams per liter of said composite dispersion medium.

10. A liquid developer for developing electrostatic latent images consisting essentially of:

15 a. a composite dispersion medium consisting essentially of i. a liquid carrier of a dielectric constant of not more than 3 and a resistance of not less than 10 ohm-centimeter selected from straight chain aliphatic hydrocarbons, branched chain aliphatic hydrocarbons, halogenated hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, mixed'hydrocarbon liquids and dimethyl polysiloxane, and

ii. an extract of asphalt which is at least substantially completely dissolvable in said liquid carrier, said extract of asphalt being obtained by extracting an asphalt selected from petroleum asphalt, straight-asphalt and blown asphalt with a saturated hydrocarbon liquid selected from npentane, n-hexane, n-octane, n-decane, nundecane, n-dodecane, n-tridecane,- nhexadecane, n-octadecane, isohexane, 2,2- dimethylbutene, isooctane, C -C isoparaffin mixture (boiling point 176.7 180C.) and C -C, paraffin mixture (boiling 159 176.7C. cyclopentane and methylcyclohexane, and

b. an image-forming material dispersed in said com posite dispersion medium selected from dyestuffs,

' pigments and powdery metals, said image-forming material being present in an amount of 0.05 to 5 grams per liter of said composite dispersion medium, said extract of asphalt being present in an amount of 0.1 to 30 grams per liter of said composite dispersion medium, the particles of said imageforming material being negatively charged in said composite dispersion medium.

11. The liquid-developer of claim 10 wherein said extract of asphalt is present in an amount of 0.5 3 grams per liter of said composite dispersion medium.

12. The liquid developer of claim 10 wherein said image forming material is present in an amount of from 0.1 to 1.5 grams per liter of said composite dispersion medium. g

13. The liquid developer of claim 10 wherein the extraction of said asphalt is carried out by using 5 to 1000 parts of said saturated hydrocarbon liquid per 1 part of said asphalt.

i UNITED STATES PATENT PFFICE CERTIFICATE OF CORREC'IION Patent NO. 3,821,113 Dated June 28, 1974 Hiroichi MORIKAWA ET AL Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Heading, insert patentees' claims for priority as follows: Claims priority, application Japan, No. 45/14143,

February 20, i970. v

Signed and mm this 25th day of October 1974.

(SEAL) Attest:

McCOY M. GIBS ON JR; 0. MARSHALL DANN Arresting Officer Commissioner of Parents FORM PC4050 (10-69) USCOMM DC (ma-Imp 1% us, GOVERNMENT PRINTING OFFICE: 1959 o-aea-au 

2. The liquid developer of claim 1 wherein said straight chain aliphatic hydrocarbon is selected from n-pentane, n-hexane, n-heptane, n-octane, n-decane, n-undecane, n-dodecane, n-tridecane, n-hexadecane and n-octadecane.
 3. The liquid developer of claim 1 wherein said branched aliphatic hydrocarbon is selected from isopentane, isohexane, 2, 2-dimethylbutane, isooctane, and C9-C12 isoparaffin mixtures (boiling point 176.7 - 186*C.), C9-C12 paraffin mixtures (boiling point 159 - 176.7*C.) and Isopar R.
 4. The liquid developer of claim 1 wherein said alicyclic hydrocarbon is selected from cyclopentane, cyclohexane and methylcyclohexane.
 5. The liquid developer of claim 1 wherein said straight chain aliphatic hydrocarbon contains from 5 to 16 carbon atoms, said branched chain aliphatic hydrocarbon contains from 5 to 12 carbon atoms and said alicyclic hydrocarbon contains 5 or 6 carbon atoms.
 6. The liquid developer of claim 1 wherein said liquid carrier is selected from straight chain aliphatic hydrocarbons, branched chain aliphatic hydrocarbons, halogenated hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, mixed hydrocarbon liquids and dimethyl polysiloxane.
 7. The liquid developer of claim 1 wherein said extract of asphalt is present in an amount of 0.5 - 3 grams per liter of said composite dispersion medium.
 8. The liquid developer of claim 1 wherein said image forming material is present in an amount of 0.05 to 5 grams per liter of said composite dispersion medium.
 9. The liquid developer of claim 8 wherein said image forming material is present in an amount of from 0.1 to 1.5 grams per liter of said composite dispersion medium.
 10. A liquid developer for developing electrostatic latent images consisting essentially of: a. a composite dispersion medium consisting essentially of i. a liquid carrier of a dielectric constant of not more than 3 and a resistance of not less than 109 ohm-centimeter selected from straight chain aliphatic hydrocarbons, branched chain aliphatic hydrocarbons, halogenated hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarbons, mixed hydrocarbon liquids and dimethyl polysiloxane, and ii. an extract of asphalt which is at least substantially completely dissolvable in said liquid carrier, said extract of asphalt being obtained by extracting an asphalt selected from petroleum asphalt, straight asphalt and blown asphalt with a saturated hydrocarbon liquid selected from n-pentane, n-hexane, n-octane, n-decane, n-undecane, n-dodecane, n-tridecane, n-hexadecane, n-octadecane, isohexane, 2,2-dimethylbutene, isooctane, C9-C12 isoparaffin mixture (boiling point 176.7 - 180*C.) and C9-C12 paraffin mixture (boiling 159 - 176.7*C.), cyclopentane and methylcyclohexane, and b. an image-forming material dispersed in said composite dispersion medium selected from dyestuffs, pigments and powdery metals, said image-forming material being present in an amount of 0.05 to 5 grams per liter of said composite dispersion medium, said extract of asphalt being present in an amount of 0.1 to 30 grams per liter of said composite dispersion medium, the particles of said image-forming material being negatively charged in said composite dispersion medium.
 11. The liquid developer of claim 10 wherein said extract of asphalt is present in an amount of 0.5 - 3 grams per liter of said composite dispersion medium.
 12. The liquid developer of claim 10 wherein said image forming material is present in an amount of from 0.1 to 1.5 grams per liter of said composite dispersion medium.
 13. The liquid developer of claim 10 wherein the extraction of said asphalt is carried out by using 5 to 1000 parts of said saturated hydrocarbon liquid per 1 part of said asphalt. 